move most code from method_compiler to builder

rather use builder in two ways, than sometimes compiler and sometimes
builder
Also makes it possible to reuse builtin code in mom’s to_risc, as both
use builder. The builtin code by directly adding to compiler, the mom
code not.

test/risc/test_risc_value.rb

@@ -2,7 +2,7 @@ require_relative "../helper"
 
 class FakeBuilder
   attr_reader :built
-  def add(ins)
+  def add_code(ins)
     @built = ins
   end
 end